[postgres] support some of libpg's ENV variables when connecting, namely:
- PGHOST
- PGPORT
- PGUSER
- PGDATABASE
- PGPASSWORD
- PGCONNECT_TIMEOUT
NOTE: that jdbc generator is not needed (will only show up on Rails >= 3.0)
do not load deprecated file from a deprecated one
warn on double loading AR-JDBC's databases.rake (smt of a "fix" for #493)
get rid of `ActiveRecord::VERSION::MAJOR < 2` code